The Padres also signed right-hander Craig Stammen to a one-year contract. He'll begin as a reliever, and Robbie Erlin (Tommy John surgery) was moved to the 60-day disabled list to make room for Stammen on the 40-man roster.
And finally, Carter Capps, also barely a year out from Tommy John surgery, will open on the 10-day DL, but he will travel with the team to Los Angeles where the Padres open against the Dodgers on Monday.
Capps is slated to pitch a simulated game at Dodger Stadium and then a determination will be made where he will begin his rehab assignment, Green said. The Minor League season opens later next week.
The rotation is now as follows: Jhoulys Chacin gets the season opener against the Dodgers, with Clayton Richard, Cahill and newcomer Jered Weaver following in the four-game series at Los Angeles. Perdomo is slated for the home opener at Petco Park against the Giants on April 7.
Despite a decent spring, Jarred Cosart will open in the 'pen. But the rotation is fluid, Green insisted.
"From my perspective, it was a very good battle for those last two spots," Green said. "It could easily have gone Jarred Cosart's way the way he threw the ball in camp. Long-term, he could very easily ascend into a starting role. He's got the mindset now where he's going to embrace the bullpen. Try something new, see if there's a change in his stuff for the positive.
"For me, the way he's been in camp, the way he's thrown the baseball, it's all been really, really good."
Cahill, another newcomer, is back in a starting role for the first time since the opening of the 2014 season. He finished that season with the D-backs at 3-12 with a 5.61 ERA and a 1.61 WHIP, and he spent a stint in the Minors trying to relocate his pitches in the strike zone.
Since then, Cahill has bounced from the Braves to the Cubs, the latter where he became effective again working in middle-inning relief. He was in the 'pen last year when the Cubs won the World Series, snapping a 108-year drought.
Green said he's hoping Cahill, now 29, can regain the form of his career-best 2010 season when he finished 18-8 with a 2.97 ERA and a 1.11 WHIP in 30 starts for the A's.
"Trevor threw the ball very well in camp, too," Green said. "He's got strikeout stuff. At any point in time, it's a four-pitch mix. He's had success in the rotation before. We think he has the possibility of getting back to who he was in Oakland. And even getting beyond where he was in Oakland. There was a time when he was considered one of the best young starters in the game."
Perdomo made 35 appearances for the Padres in 2016, finishing the season with the bulk of his 20 starts. As a starter, he was 8-9 with a 4.85 ERA, 81 strikeouts and 30 walks in 117 innings. Perdomo's high-water mark was a complete-game, one-run, six-hit effort in a 3-1 victory over the Marlins on Aug. 28 in Miami.
"He's checked every box I think you can ask a young guy to check," Green said about the 23-year-old right-hander. "He's been outstanding from the first day of camp to the last day of camp. He didn't show up taking it for granted he was going to be on a Major League club because he pitched well last year."